One other parameter of concern to most TCXO users is the long-term drift of the frequency caused by aging. Although other oscillator components can contribute to aging, in a well-designed oscillator the aging is primarily due to the crystal. Changes in the crystal's resonant frequency arise because of mass transfer to or from the quartz blank. Relaxation of mounting stresses can also play a role. Advances in crystal design and processing have reduced the aging capability to less than 1 ppm per year, even for miniature packages. Long-term projections for the 10- or 20-year expected life of an oscillator can be less than 5 ppm, as the aging rate decays with time. Aging effects can be projected with curve-fit extrapolation using the MIL-SPEC logarithmic model:

f/f (t) = a₀ + a₁ln(1+a₂t)

Where t is the time in days, and a₀, a₁ and a₂ are numerical co-efficients adjusted for curve fitting to the sample data.

If the oscillator's operating environment includes vibration and shock levels, the acceleration or “g” sensitivity (where g = 9.8 m/s²) of the crystal can be an important parameter. Vibration levels will modulate the output causing noise sidebands on the signal. Shock pulses will produce short perturbations in the frequency, which may be problematic for phase locked loops or similar circuitry. The miniature AT strip crystals may be designed to provide low sensitivity to these forces. Levels below 5 × 10^-10 (or 5×10^-4 ppm) per g in the worst axis are routinely produced for critical applications. Because of the design of the strip resonator and its mount, the worst axis for acceleration is predictable. The vector always points in the vertical or z-axis, almost directly perpendicular to the crystal plate. The sensitivity in the x and y axes is extremely low. These crystals can also withstand high levels of pyrotechnic shock. Some have been tested to 100,000 g.⁸

Since the basic TCXO architecture has been integrated into a single IC, which is suitable for many applications, further reductions in the size of precision oscillators will require smaller resonators. Although bulk-mode quartz resonators can be made small, physical limitations preclude making usable devices below a certain size. Surface-mount packages with 3.2 mm × 5 mm or smaller footprints (Figure 7) are available with reasonable motional parameters and stabilities. But reductions much beyond this level may require advancement of resonator technologies. Silicon micro-machined resonators can be fabricated on the same die as the oscillator circuitry^[9]. Although these oscillators have not achieved the stability of a precision TCXO, further improvements are directed toward this goal. These devices may soon begin to displace quartz oscillators in lower-end, high-volume applications, but quartz crystals will still be required for precision frequency control for the foreseeable future.

1. Cady, W.G. “Piezoelectricity,” New York, McGraw-Hill, 1946.

2. Gerber, E.A. and Sykes, R.A., “A Quarter Century of Progress In the Theory and Development of Crystals for Frequency Control and Selection,” Proceedings of the 25^th Annual Symposium On Frequency Control, 1971, pp. 1-45.

3. Wood, A.F.B. and Seed, A., “Activity Dips In AT Cut Crystals,” Proceedings of the 21^st Annual Symposium On Frequency Control, 1967, pp. 420-435.

4. Frerking, M.E., “Fifty Years of Progress In Quartz Crystal Frequency Standards,” Proceedings of the 50^th Annual Symposium on Frequency Control, 1996, pp. 33-46.

5. Fry, S.J., “Temperature Compensation Using a Voltage Tuned Thermistor Network,” Proceedings of the Quartz Devices Conference, 1998.

6. Buroker, G.E. and Frerking, M.E., “Digitally Compensated TCXO,” Proceedings of the 27^th Annual Symposium On Frequency Control, 1973, pp. 191-198.

7. Kubo, K. and Shibuya, S., “Analog TCXO Using One Chip LSI for Mobile Communication,” Proceedings of the 50^th Annual Symposium on Frequency Control, 1996, pp. 728-734.

8. Statek Corporation, HGXO datasheet.

9. Hsu, W.T., “Reliability of Silicon Resonator Oscillators,” Proceedings of the 2006 International Frequency Control Symposium.

Steve Fry is the development engineering manager at Greenray Industries in Mechanicsburg, Pa. After receiving a BSEET from the Ohio Institute of Technology, he has been involved in the design and development of frequency control devices for more than 25 years. In addition to writing various papers, Fry holds 10 U.S. patents. He may be reached at sfry@greenrayindustries.com.

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